Fix isNullableClass to also work after Erasure

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -496,13 +496,18 @@ object SymDenotations {
      */
     def derivesFrom(base: Symbol)(implicit ctx: Context) = false
 
-    /** Is this symbol a class that does not extend `AnyVal`? */
-    final def isNonValueClass(implicit ctx: Context): Boolean =
-      isClass && !derivesFrom(defn.AnyValClass) && (symbol ne defn.NothingClass)
+    /** Is this symbol a class that extends `AnyVal`? */
+    final def isValueClass(implicit ctx: Context): Boolean = {
+      val di = this.initial.asSymDenotation
+      di.isClass &&
+      di.derivesFrom(defn.AnyValClass)(ctx.withPhase(di.validFor.firstPhaseId))
+        // We call derivesFrom at the initial phase both because AnyVal does not exist
+        // after Erasure and to avoid cyclic references caused by forcing denotations
+    }
 
     /** Is this symbol a class references to which that are supertypes of null? */
     final def isNullableClass(implicit ctx: Context): Boolean =
-      isNonValueClass && !(this is ModuleClass)
+      isClass && !isValueClass && !(this is ModuleClass)
 
     /** Is this definition accessible as a member of tree with type `pre`?
      *  @param pre          The type of the tree from which the selection is made

src/dotty/tools/dotc/transform/ValueClasses.scala

@@ -12,13 +12,9 @@ import Flags._
 object ValueClasses {
 
   def isDerivedValueClass(d: SymDenotation)(implicit ctx: Context) = {
-    val di = d.initial.asSymDenotation
-    di.isClass &&
-    di.derivesFrom(defn.AnyValClass)(ctx.withPhase(di.validFor.firstPhaseId)) &&
-      // need to check derivesFrom at initialPhase to avoid cyclic references caused
-      // by forcing in transformInfo
-    (di.symbol ne defn.AnyValClass) &&
-    !di.isPrimitiveValueClass
+    d.isValueClass &&
+    (d.initial.symbol ne defn.AnyValClass) && // Compare the initial symbol because AnyVal does not exist after erasure
+    !d.isPrimitiveValueClass
   }
 
   def isMethodWithExtension(d: SymDenotation)(implicit ctx: Context) =

tests/pos/nullAsInstanceOf.scala

@@ -0,0 +1,11 @@
+object Test {
+  val b = null.asInstanceOf[Byte]
+  val c = null.asInstanceOf[Char]
+  val s = null.asInstanceOf[Short]
+  val i = null.asInstanceOf[Int]
+  val l = null.asInstanceOf[Long]
+  val f = null.asInstanceOf[Float]
+  val d = null.asInstanceOf[Double]
+
+  val str = null.asInstanceOf[String]
+}